The technical field of this invention is that of nondestructive materials characterization, particularly quantitative, model-based characterization of surface, near-surface, and bulk material condition for flat and curved parts or components. Characterization of bulk material condition includes measurement of changes in material state, such as degradation/damage caused by damage or thermal exposure; (2) inspection for the presence of flaws or defects, such as the presence and extent of porosity or a delamination; and (3) assessment of processing-related conditions, such as cure state. Characterization of surface and near-surface conditions includes measurements of surface roughness, displacement or changes in relative position, coating thickness, temperature and coating condition. The characterization uses an electromagnetic field to interrogate a semiconducting or insulating material of interest to deduce physical, geometric, or kinematic properties.
Dielectric sensors are commonly used for material property characterization and defect detection in a material under test (MUT). The sensors respond to the absolute properties of the MUT, such as the electrical permittivity, electrical conductivity, thickness, and proximity, and changes in those properties. Factors that affect the dielectric properties include the state of cure, density, porosity, and contamination with other substances such as moisture. The property variations may be a normal part of the manufacturing process or a result of the presence of defects or damage. These defects can be created during the manufacturing process, such as improper curing or incorrect layer thickness for stratified media, or when the material is placed into service by age-related degradation processes, such as fatigue. In manufacturing, the continuing drive toward defect-free products, yield improvement and operation near the capability limits of the production system require sensing technologies for monitoring as many critical process variables as possible. In operations, service maintenance, and repair and replacement activities, the continuing push toward a retirement-for-cause philosophy from the retire-for-time approach requires reliable measurements on all fatigue-critical components in the system, even at difficult-to-access locations.
Dielectric measurements can be performed with a wide variety of devices. The simplest devices involve parallel plate capacitors where the electrodes sandwich the MUT. Often guard electrodes are used to minimize the effects of fringing electric fields at the electrode edges so that MUT is exposed to an essentially uniform electric field. The electrical terminal admittance or impedance of the device is then related to the material properties through geometric factors associated with the sensor geometry.
In many applications both sides of the MUT are not easily accessible and single-sided sensor configurations are required. A common implementation of a single-sided sensor is the interdigitated electrode structure used for chemical and moisture sensing applications (U.S. Pat. No. 4,423,371 and Sheppard et al, Sensors and Actuators, vol. 2, pp. 263-274, July, 1982). U.S. Pat. No. 4,814,690 further discloses the use of multiple sets of interdigitated electrodes as part of the imposed frequency-wavenumber dielectrometry approach for spatial profiling of stratified dielectric media. These devices have been effective in determining the dielectric properties of fluids. However, the determination of solid dielectric properties at multiple locations or over the surface of a test material is more difficult because of the presence of microcavities and unintentional or varying air gaps between the solid dielectric and the sensor.